the reason for the need to have extra batting/backing is because as it is quilted the fabric (draws up) just like when you embroider or applique (which is why we cut our squares to be embroidered/appliqued larger-then trim to size)
it also allows for the chance of things not being exactly square- after the quilt is quilted the batting/backing is trimmed to size-the quilt can then be squared.
if your backing/batting is the same size as your top by the time you are reaching the end of the quilt chances are you will have top left-and no backing/batting
it also takes into account the top and bottom being attached to the leaders- loaded for long-arm quilting-